The invention is in the field of thermal wave imaging/sensing and characterization for nondestructive/noncontact evaluation. Specifically, the invention comprises the use of microwave heating with Time Resolved Infrared Radiometry (TRIR) methods.
TRIR is a thermal characterization technique developed for the nondestructive evaluation of layered materials. In TRIR a region close to a sample""s surface is heated by a source, e.g., a laser or flashlamp, with a long pulse and the sample""s surface temperature is monitored as a function of time through changes in emitted infrared radiation. Specimen features which influence the production or transport of heat cause the surface temperature to change relative to areas without such features. This has allowed subsurface delaminations to be imaged.
An infrared imaging camera allows rapid, quantitative inspection at relatively high spatial resolution. However, the visibility of the subsurface specimen features in the thermal image is determined by the magnitude of the reflected thermal signal which is determined by the depth of the defect and the ratio of the thermal effusivities of region and sample. For example, for subsurface voids filled with water this contrast is small.
In the method of the invention, a specimen/sample of a material is illuminated/heated with microwaves and then a temperature imaging/sensing means/method, such as an infrared imaging device (e.g., focal plane array), monitors the heating in the specimen due to electrical and/or magnetic property discontinuities, e.g., dielectric loss or the presence of a conducting contaminant.
For optically opaque but microwave transparent materials containing localized absorbing regions the use of a microwave heating source, when compared with conventional laser or flashlamp sources, has distinct advantages. For particular specimen geometries and material properties, the presence of the defect region can be imaged at higher contrast and better spatial resolution than for the surface heating case of TRIR, hence, enhancing the detectability of such defect regions. Since the temperature has only to diffuse to the surface, the characteristic thermal transit times for the measurement are shorter. Moreover, since three-dimensional diffusion acts as a spatial low pass filter and reduces the image resolution of localized thermal features, the shorter path for the thermal signal allows better resolution. Finally, when the region of interest can be selectively heated by specific microwave wavelengths, the image contrast is enhanced.